We also point out that usually, the curl \(\nabla \times {\operatorname{u}}\)  is enough to quantify the vorticity of the fluid. However, as pointed out by Kasumba, K. and Kunisch, K. in [1]}, the magnitude of such quantity may still be high on laminar flows. As such, on the same literature the authors proposed the second term in the vortex functional for the quantification of rotation of fluids. The impetus is that the rotational cores of fluids mostly occur near regions where the eigenvalues of \(\nabla {\operatorname{u}}\)  are complex [2]}, [3]}, [4]}, and that the eigenvalues are complex when \(\mathrm {det}\nabla {\operatorname{u}} > 0\) .
